Traditional ways to produce transparent paper involve fiber-based and sheet processing techniques. Fiber-based methods use overbeaten wood pulp, while sheet processing requires coating, impregnating, supercalendering, or chemical immersion to produce transparent paper. These methods consume large amounts of energy or rely on petroleum-based materials to produce paper with no more than 80% transmittance. Since Herrick and Turbak successfully separated nanofibers from wood pulp using a mechanical process in a high pressure homogenizer in 1983, cellulose nanofibers have attracted great attention because they can be used to manufacture transparent paper for printed electronics, optoelectronic devices, and also for packaging. Related art transparent paper is made of NFCs (nanofibrillated cellulose) which involves a fabrication process that is too time and energy consuming to be practical for commercial applications.
Some related art techniques are used to liberate nanofibers. These techniques include mechanical treatments and acid hydrolysis. Mechanical treatment techniques are currently considered efficient ways to isolate nanofibers from the cell wall of a wood fiber. However, solely mechanical processes consume large amounts of energy and insufficiently liberate the nanofibers while damaging the microfibril structures in the process. Pretreatments, therefore, are conducted before conducting mechanical disintegration in order to effectively separate the fibers and minimize the damage to the nanofiber structures.
TEMPO-mediated oxidation is proven to be an efficient way to weaken the interfibrillar hydrogen bonds that facilitate the disintegration of wood fibers into individualized nanofibers yet maintain a high yield of solid material. Nanopaper made of nanofibers can attain a transmittance of over 80%, yet this type of transparent paper takes a longer time to fabricate and has a very low haze.
Solar cell substrates require high optical transparency, but also prefer high optical haze to increase the light scattering and consequently the absorption in the active materials. Common transparent paper substrates generally possess only one of these optical properties, which is exemplified by common transparent paper substrates exhibiting a transparency of about 90% yet a low optical haze of <20%.
Substrates play a key role as to the foundation for optoelectronic devices. Mechanical strength, optical transparency, and maximum processing temperature, are among the critical properties of these substrates that determine its eligibility for various applications. The optoelectronic device industry predominantly utilizes glass substrates and plastic substrates for flexible electronics; however, recent reports demonstrate transparent nanopaper based on renewable cellulose nanofibers that may replace plastic substrates in many electronic and optoelectronic devices. Nanopaper is entirely more environmentally friendly than plastic substrates due to its composition of natural materials; meanwhile it introduces new functionalities due to NFCs' fibrous structure.
The maximum transparency among all current reports on glass, plastic, and nanopaper substrates is about 90%, but with a very low optical haze (<20%). Optical haze quantifies the percent of the transmitted light that diffusely scatters, which is preferable in solar cell applications. Optical transparency and haze are inversely proportional values in various papers. Trace paper has a high optical haze of over 50%, but a transparency of less than 80%; whereas plastic has a transparency of about 90%, but with an optical haze of less than 1%. Related art Nanopaper based on NFCs has the highest reported optical haze among transparent substrates due to its nanoporous structure, yet it is still a relatively low value.
Although optical haze is a property preferably maximized in transparent substrates integrated into solar devices, other optoelectronic devices require different levels of light scattering; for example, displays and touch screens need high clarity and low optical haze. Plastic substrates have been widely used as a flexible material for packaging, electronics and other applications. Transparent paper was recently demonstrated, but the optical haze is too large for most of the optoelectronics applications. In devices such as organic light emitting diodes, liquid crystal display TVS and others, the optical haze has to be less than 1%. Transparent paper developed before has an optical haze >10%, which is too high for the above-mention applications.
Current commercial substrates are best suited for displays, but are not optimized for solar cell devices because of the low optical haze. Various materials such as SiO2 nanoparticles or silver nanowires are reported to effectively increase light absorption and consequentially the short-circuit current by enhancing the path of light through the active solar layer with increased diffuse light scattering. The light scattering induced by these nanostructures is limited, however, and incorporating these materials requires additional steps that add cost to the solar cells devices.